Sayyiduna Abu Hurayrah (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "Whatever part (of the body) is covered by the lower garment below the ankles will burn in the Fire." These are the words of Adam's narration; the words of Abd al-Samad's narration are: "Whatever of the lower garment is below the ankles will burn in the Fire."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3318